Many traditional solvent-based cleaning applications can suffer from poor performance on aqueous born soils. A significant portion of the soils found in conventional dry cleaning can be categorized as partially or wholly water-soluble. Water-in-oil surfactants have been developed that effectively disperse water to yield optically clear homogeneous mixtures. These dispersions can effectively dissolve water-soluble soils, termed secondary solublization, if the proper water activity is achieved in a given cleaning solvent. Water activity, determined by a number of factors including temperature, the nature of solvent-solute interactions and the molar ratio of surfactant to water, is generally monitored in conventional dry cleaning by what is termed as relative humidity. A cleaning bath with low relative humidity and hence low water activity will not allow for secondary solublization of aqueous born soils. Water exceeding a critical level can lead to non-dispersed bulk water that can be deleterious to certain garment types.
Carbon dioxide based dry cleaning is a new technology that has only recently been commercially implemented. Like conventional dry cleaning solvents water-soluble soils are not inherently soluble in liquefied carbon dioxide. Surfactant systems that enable the water bearing nature of liquid carbon dioxide have been disclosed in the patent and open literature. Under certain conditions these systems have demonstrated that water-soluble materials can be dissolved and dispersed in a liquid carbon dioxide medium.
Many conventionally used water-in-oil surfactants applied to dry cleaning solvents are not compatible with liquid CO.sub.2 solvent systems. Surfactants containing what is termed to be "CO.sub.2 -philic" function have been proven to be useful in the emulsification of water in CO.sub.2. The exclusive use of some of these materials can be cost prohibitive for many applications. The case for dissolution of water-soluble materials in CO.sub.2 can be further complicated by the reversible reaction between water and carbon dioxide producing carbonic acid. This weak acid which reverts back to water and carbon dioxide as pressure is lowered and CO.sub.2 is removed can have substantial implications on water activity in CO.sub.2. Lower water activity can effect the ability of the CO.sub.2 cleaning fluid to dissolve water-soluble soils. Certain pH buffers have been used in liquid and supercritical CO.sub.2 to control the pH of aqueous micro and macro-domains and in turn augment water activity. Attempts to raise the water activity in current processes by the addition of bulk water can fail because of the inability of the CO.sub.2 and surfactant combinations to sufficiently stabilize the water. Bulk water phase-separated from liquid CO.sub.2 cleaning fluids and conventional cleaning fluids can have substantial detrimental effects on many dry clean only fabrics.
Not all stains are water soluble. Indeed, a significant number of stains that must be cleaned in a dry cleaning operation are hydrophobic. Thus, in addition to aqueous detergent formulations, it is also desirable to have a means for adding low water content detergent formulations to carbon dioxide dry cleaning systems.
U.S. Pat. No. 5,858,022 to Romack et al. and U.S. Pat. No. 5,683,473 to Jureller et al. (see also U.S. Pat. No. 5,683,977 to Jureller et al.) describe carbon dioxide dry cleaning methods and compositions. Our co-pending U.S. patent application Ser. No. 09/047,013 of McClain et al., filed Mar. 24, 1998, describes carbon dioxide dry cleaning apparatus. Dry cleaning apparatus is also described in U.S. Pat. No. 5,467,492 to Chao et al. U.S. Pat. No. 5,651,276 to Purer et al., and U.S. Pat. No. 5,784,905 to Townsend et al. It will be seen that there is a need for better ways to add detergent formulations to the carbon dioxide during operation of the apparatus.